Pictures Considered. The E. coli Chromosome Caught in the Act of Replicating

We continue our series of images that have made a difference in microbiology. This one, published by John Cairns in 1963 has earned a most deserved place in textbooks and reviews. It shows a radioautograph of an intact E. coli chromosome in the act of replicating. From this image, Cairns concluded that the E. coli chromosome is:
a single piece of double stranded DNA, 700-900 μm long, and
circular.
These truths had been surmised from other, mainly indirect pieces of evidence. For instance, the idea that the genome consists of a single chromosome had already been inferred from the fact that only a single genetic linkage group could be found in E. coli. Its circular nature had been suspected because all the genes on the chromosome were circularly permuted. But this image gave these notions physical authenticity, ergo finality. Such important conclusions from one single picture!

In addition, by labeling the newly replicated DNA in the chromosome with a short pulse of tritiated thymidine, Cairns provided conclusive evidence for the bidirectionality of chromosome replication, with each of the two replicating forks containing one new and one old DNA strand.